An image sensor such as a complementary metal-oxide-semiconductor (CMOS) image sensor and a method of manufacturing the same are provided. The CMOS image sensor includes: a semiconductor substrate including a first surface and a third surface formed by removing a part of the semiconductor substrate from a second surface opposite to the first surface; a plurality of active regions which are formed between the first surface and the third surface and each of which includes a photoelectric conversion element generating charges in response to light input through the third surface; and an isolation region vertically formed from either of the first and third surfaces to isolate the active regions from one another. When the CMOS image sensor is viewed from the above of the third surface, each of the active regions may have round corners and concave sides.